1. Field of the Invention
The present invention concerns a method of manufacturing soft magnetic Fe-Si alloy sintered products capable of obtaining products of excellent soft magnetic property and dimensional accuracy upon sintering.
2. Description of the Prior Art
Fe-Si series alloys have been used widely as magnetic materials, for example, as Fe-3% Si alloys have been often used for head yoke materials of dot printers.
Generally, addition of Si to Fe can increase magnetic permeability and electric resistance and improve AC magnetic property. However, as Si addition amount is increased, since the material becomes hard and fragile, it is difficult to apply plastic fabrication or grinding, to remarkably reduce the fabrication yield. Therefore, in a case of manufacturing, for example, a head yoke of a complicate shape through the steps of melting, casting and fabrication, the production cost is increased.
Then, for compensating such a drawback, products of complicate shapes have now been manufactured usually by using a precision casting process of using a die made of ceramics of a predetermined shape, casting a molten solution of an Fe-Si alloy into the die, cooling and then taking out the product from the die. However, in the precision casting method, since metal is melted and cast into a desired shape, segregation may be caused upon solidification or large pores may remain, making it difficult to stably produce products of excellent soft magnetic property.
For overcoming such a drawback, it has been attempted recently to produce Fe-Si alloy parts by means of powder metallurgy. However, since Si powder or Fe-Si alloy powder is hard, compacting is difficult in the usual powder metallurgy, even applying a great pressure upon compacting process and cracks tend to occur. In order to solve this problem, there has been proposed a method of dispersing one or both of fine Si powder or Fe-Si alloy powder with the mean particle size of less than 44 .mu.m in a relatively coarse Fe powder such as having a mean particle size of from 40 to 100 .mu.m, to obtain an aimed composition, thereby improving the compressibility (Japanese Patent Laid-Open Sho No. 62-27545, etc.).
However, if it is intended to maintain the dimensional accuracy when the molding product so called "green body" obtained by the improved dispersion method described above is sintered, final relative density after sintering can be increased only to about 90% at the highest and, in addition, diffusion of Si into Fe powder is insufficient making the distribution of Si not homogenous since coarse Fe powders of 44 to 100 .mu.m is used. Since the soft magnetic property is deteriorated as the porosity is high and the Si distribution is not (more) uniform, there has been a problem that the sintered products from the green body by the method described above is remarkably poor as compared with those obtained by the melting process conducted so far.